1. Field of the Invention
The present invention relates to a method of driving a solid-state image pickup device, in particular, a solid-state image pickup device of the frame transfer type.
2. Description of the Prior Art
As an example, in case of using the solid-state image pickup device of the frame transfer type (e.g., a CCD) for the purpose of image pickup in a television, it is necessary to drive such a device at the same period as the field period of the television. For instance, in the television system compliance with the NTSC system, the field period is 1/60 sec. Therefore, in case of the frame transfer type device, it is necessary to drive this so as to transfer the signal (charges) from the image sensing part to the storage part at a period of 1/60 sec and to read out the stored signal (charges) from the storage part for almost 1/60 sec.
On the other hand, although it is also necessary to set the output of the device or the level of the image pickup signal to be obtained by processing this output to be within a predetermined stable range for the wide luminance variation of an image object to be picked up, this point is solved in the manner such that, for example, a quantity of incident light into the image sensing part is adjusted using a diaphragm or other means, or that an AGC amplifier is interposed in the output processing system, thereby adjusting the signal level. However, in most cases where an image object is extremely bright, in many cases the signal level cannot be sufficiently adjusted even by the above-described diaphragm or AGC amplifier.
In addition, for example, in other cases where the object is moving at a high speed, the driving of the device for the time period in accordance with the field period of the television mentioned above causes the photo-sensing time by the image sensing part to be almost 1/60 sec; therefore, this may cause an undesirable situation such as, so-called, flow or slide of picture image upon display.
In any of the above cases, such inconvenience is caused due to the fact that the device has to be driven for the time period in accordance with the field period of the television. To avoid this inconvenience, a method has been considered whereby a mechanical shutter is used, thereby controlling the photo-sensing time. However, an additional shutter must be provided, so that the image pickup apparatus becomes large and the constitution and control method becomes complicated; moreover, it will cause a new drawback in the increase in price of the apparatus.
On the other hand, for example, according to the specification of U.S. Pat. No. 3,944,816, in a method of driving a solid-state image pickup device in which a storage pulse (i.e., photo gate bias) is added to the image sensing part to form a potential well and the signal (charges) generated due to the photo-excitation is stored (are accumulated) in this well, there is disclosed a driving method whereby the photo-sensing interval corresponding to one field interval of the television is divided into the non-storage interval of the length responsive to the intensity of the incident light and the storage interval. The storage pulse is added for only the storage interval, thereby storing the generated signal into the well thus formed. According to this method, it is possible to avoid such drawback as mentioned before.
However, even in this method, a problem still remains which has to be solved. Namely, the signal (charges) is generated by the image sensing part due to the photo-excitation even during the above-mentioned non-storage interval. This state is the diffused state over the whole image pickup surface of the image sensing part since the storage well is not formed because no storage pulse is added. Therefore, thereafter when the well is formed by adding the storage pulse in the storage interval, the signal which is in the diffused state is stored in a proper well. Consequently, this signal in the diffused state exerts the influence, as the noise, on the normal signal to be obtained in the storage interval after that, so that this causes the picture quality to largely deteriorate. It is also of course possible, for instance, to provide a blooming preventing mechanism in the image sensing part and thereby to clear the signal generated in the non-storage interval by means of this blooming preventing mechanism. However, even in this case, there is no guarantee at all that all the signals generated can be reliably cleared by this blooming preventing mechanism, so that those signals may remain as the noise level.